17th March 2007, 17:07
Can someone who has used a Nicol prism lens, refresh my memory ? . My memory tells me that the sun's limb was centred exactly between the true and reflected horizons. I never owned one but sailed with an old man who lent me his.
17th March 2007, 19:47
As I recall from over 50years ago the Nicol's prism was inserted in the telescope with it's plane parallel to the plane of the sextant. The glare was then refracted out giving a true and good horizon free of glare at low altitudes and not requiring the error inducing use of the horizon shades.
Hope this may stimulate the old grey matter and you will be able to clarify the subject for us all
19th March 2007, 14:33
Thanks David. Have searched Google and Bowditch without success as regards sextant use of this lens. Wonder now if there was a variation which gave a reflected horizon. The lens screwed/clipped on to the eyepiece of the telescope. This was a handy gadget on radarless ships when land was near and decent sights were few.
19th March 2007, 18:00
Remember buying war surplus RAF Mk VI bubble sextant for a few bob and using it at sea with reasonable results taking stars at night when no visible horizon. The bubble could be reduced in size making it suitable for stars and remained quite steady if placed on or against steadying part of the bridge even in bad weather i.e. WNA, low sun and intense cloud at twilight. This was my only experience false horizon in a reflected prism.
19th March 2007, 21:58
One time using the prism in hazy weather, standing on the fore deck, a cadet asked me how come I could see the sun and he couldn't. Fortunately for his sensibilities, I was speechless, as I had an Acme Thunderer between my teeth to communicate with the old man in the chartroom. My only war surplus purchase was an ex USN Rude star identifier. Many moons ago, a Woolworth style store in Victoria BC had ex RCN sextants on sale for a bargain price, unfortunately they were uncased, piled on top of each other, no doubt countless errors inflicted on them. Although I only had a borrowed one I gave those ones the go by.